6 BRITISH INSECTS 



image has been discarded. It is now believed, for 

 good and sufficient anatomical reasons, that each facet 

 of the eye registers in the brain not a complete picture, 

 but merely the small part of the picture which happens 

 to be exactly opposite the centre of the lens, and that 

 all these fragmental images combine in the retinal 

 region of the eye. A mosaic pavement, composed of 

 numberless pieces of stone or marble in close contact, 

 helps us to grasp the idea. Indeed, we may take it 

 as highly probable that an insect's visual perception 

 of an object does not differ greatly from our own, 

 although we know very little concerning the actual 

 power and extent of any insect's sight. 



Behind the insect's head, to which it is joined by a 

 slender " neck," is the thorax, which often appears to 

 be one piece, but is, in reality, composed of three 

 closely fitting rings, or segments, termed pro-, meso-, 

 and metathorax respectively. Each segment of the 

 thorax carries a pair of jointed legs, which vary 

 greatly in form in different kinds of insects. In 

 almost every instance, however, the five principal 

 parts of each leg may be easily distinguished. The 

 haunch, or coxa, fits into a socket in the thorax ; then 

 follows a small joint called the trochanter, which is 

 sometimes divided into two ; after this comes the 

 thigh, or femur ; then the shin, or tibia; and finally 

 the foot, or tarsus, which may consist of from one to 

 five joints, the usual number being five. The last, or 

 distal joint of the tarsus, is usually furnished with a 

 pair of claws. It is worth while fixing these technical 

 terms firmly in the mind, i because they have a much 

 more definite meaning than any possible English 

 equivalents. The tarsus of an insect, for example, is 

 something very different from a " foot " in the ordinary 

 sense of the word. 



